The male fertility genes of Drosophila melanogaster will be examined in restricted regions of the x-chromosome. Our studies indicate that male-sterile (ms) mutations are induced at a much higher rate in region 18F-20 of the polytene x-chromosome by EMS than in 9Fl-10F3 or 1Al-2Cl. These mutations, and others to be isolated, will be mapped by duplication and deletion mapping and by standard crossover methods. The sperm of ms mutants will be analyzed by cytological methods to determine the stage of sperm development or function that is genetically blocked. X-linked ms mutations will be tested for allelism with lethal mutations and ms mutations induced in Y-derivative chromosomes. We plan to determine 1) if each male fertility gene is essential for a specific process in sperm development or function; 2) if male fertility genes, in some cases, facilitate a vital function; and 3) if genes for male fertility are concentrated in the proximal region of the x-chromosome.